The table below illustrates a standard configuration Mini³ amplifier.
This configuration has a voltage gain of 5 (14dB). If you are building
a Mini³ for use with the
χ1 battery management board,
some parts are to be omitted. See below and the χ1 website
Please refer to the sections below for specific parts
recommendations and options. For parts not offered by
AMB audio shop,
(for Europe, UK and other countries), or
another vendor's stock numbers for the recommended parts are listed
for your convenience.
PCB, L1L, L1R, L1G, B+*, B-*
Mini³ printed circuit board (includes three ferrite beads and one set of board-mount battery contacts)
AMB audio shop
Alps RK0971221Z05 10KΩ stereo potentiometer with SPST switch
AMB audio shop
R1L, R1R, R1G, R3L, R3R, R4G
miniature 1% metal film resistor 330Ω
miniature 1% metal film resistor 100KΩ
miniature 1% metal-film resistor 1.5KΩ
miniature 5% or 1% carbon-film or metal-film resistor 6.2Ω
Farnell 9343504 (6.8Ω)
miniature 1% metal-film resistor 5.6KΩ
miniature 1% metal-film resistor 10KΩ
miniature 1% metal-film resistor 75Ω
solid dipped tantalum capacitor 10µF 25V
aluminum electrolytic capacitor 470µF 16V
667-EEU-FR1C471, 647-RNE1C471MDN, 647-UHE1C471MPD or 647-UPW1C471MPD6
P14394-ND, 493-3814-1-ND, P10247-ND or P12376-ND
Farnell 1219462 or 1800634
C4, C6, C7+, C7-, C8+, C8-
multilayer ceramic X7R capacitor 0.1µF 50V
Farnell 1141775 or 1457655
aluminum electrolytic capacitor 100µF 16V
667-EEU-FR1C101, 647-RNE1C101MDS, 647-UHE1E101MED or 647-UPW1E101MED
NOTE: If you're building a Mini³ for use with the χ1 battery
management board, then the items marked with an asterisk (*) are not
needed. In addition, those parts marked with double asterisks (**) need to
be checked for height. If any of them is taller than 12mm then it
must be replaced with one that is 12mm or shorter. See the
parts list section of the
χ1 website for details.
Parts recommendations & options
To obtain the blank Mini³ circuit board and some related parts,
AMB audio shop.
You can get almost everything else from the vendors listed on the
AMB Audio DIY Page.
The following is a list of recommendations and options. Please read
through this carefully before you order any parts.
All except R5: miniature 1% metal film type
Lead spacing 0.200" (5mm)
The Mini³ circuit board is designed for miniature through-hole
axial-lead 1/8W or 1/4W resistors with a body length no longer than
4mm (the recommended Xicon, Panasonic and Multicomp resistors are
3.5mm or 3.2mm), and no wider than 1.9mm. Do not use oversized
resistors, in particular do not mount resistors vertically ("tombstone
style"). The extra lead length adds unwanted inductance which
could affect the stability of the amplifier.
Notes about R3 and R4
The voltage gain of the amplifier is determined by the ratio of
R4L and R3L (as well as R4R and R3R). The default values of
1.5KΩ and 330Ω, respectively, results in a gain of 5,
which is a good for a wide array of low and high impedance headphones.
The following table shows optimized resistor values for several
other voltage gain settings. Please do not use other values than
listed to maintain low output DC offset and ultrasonic stability.
In terms of decibels:
dB = 20 * log(Gain)
Notes about R6 and R7:
The optimum value of these resistor depends on your choice of LED. Use this
online LED resistor value calculator
to determine what value your resistor values should be.
The default resistor values work well for LEDs with rated intensity of
about 15mcd to 400mcd. If you have LEDs with much higher intensity
than that, you should increase the resistances.
Notes about R8
This resistor sets the battery charging current. The default value of
75Ω charges at a gentle 16mA, so that it will wear down the battery
cells as minimally as possible, and is safe to leave connected to the
AC adapter for extended periods of time (even when the battery is fully
charged). The drawback is longer time to charge the battery from
a totally drained state back to full charge. Depending on the capacity
of the battery, it may take 15 hours or more.
You could decrease the value of this resistor to increase the charge
current. The formula is:
I = 1.25V / R
where I is the charge current in Amperes, and R is the value of
the resistor in Ohms.
For example, if you use a 47Ω resistor, the charge current will
be increased to 27mA. This will reduce the charge time, but will
increase wear on the battery slightly. Also, depending on how
high the current is and the capacity of the battery, it may no longer
be prudent to leave the AC adapter connected indefinitely after
the battery is fully charged.
The charge current should not exceed ~35mA to prevent U1 from
Volume/Power: 10KΩ stereo potentiometer with SPST switch
If you prefer, you may use 10µF 16V solid dipped tantalum
capacitors (0.100" lead spacing, radial) in these locations.
Please note that tantalum capacitors are polarized, and there is
a "+" label located next to each of these capacitors on
the circuit board denoting the positive pin.
D1, D2, D3: 1A rectifier diode
Lead spacing 0.300" (7.5mm) 1N4001 to 1N4007 series (DO-41)
(Mouser, Digi-Key, Newark, Farnell)
D4: 12V 500mW zener diode
Lead spacing 0.300" (7.5mm) BZX55C12 or
(AMB audio shop, Mouser, Digi-Key, Newark, Farnell)
LED1, LED2: T-1 (3mm)
Lead spacing 0.100" (2.5mm)
Your choice of size and color (affects value of R6 and R7)
Notes about D2 and D3:
You may use
schottky diodes (surface mount, SMB/DO-214AA)
in these locations. The circuit board has pads to accommodate this option.
These schottky diodes have lower forward voltage drop than standard diodes.
If your front panel is non-conductive (i.e., plastic), you may use non-isolated
jacks such as
Heatsink thermal compound
This is used to help conduct heat from the U2 voltage regulator and U4/U5
opamps to the board, improving the power dissipation. Only a very small
amount is needed. Be sure to use an electrically non-conductive compound
to prevent the opamp pins from being shorted by it.
The Mini³ is designed specifically for the Hammond 1455C80x series
extruded aluminum cases.
1455C802 clear anodized with plastic end-caps (Mouser, Digi-Key, Newark, Farnell)
1455C802BK black anodized with plastic end-caps (Mouser, Newark, Farnell)
1455C801 clear anodized with aluminum end-panels (Mouser, Digi-Key, Newark, Farnell)
1455C801BK black anodized with aluminum end-panels (Mouser, Digi-Key, Newark, Farnell)
A precision-drilled, engraved aluminum
is available from AMB audio shop. Use these for ease of build,
upgraded appearance, and to assure a perfect fit. The
Mini³s shown in the Overview section
are equipped with these panels.
The Alps volume control used in the Mini³ has a 6mm (0.236")
"D" shaft, so the volume knob should be made for this and
have a single set-screw. Knobs made for a 0.25" shaft will
be slightly eccentric. The diameter of the knob should be no larger
than about 0.5" (13mm) due to the small size of the front panel
and proximity to the nearby panel screw and input jack.
Kilo International ML-50
machined and anodized solid aluminum "soft-touch" knobs are
recommended. These are high in quality and attractive. The usable
variations are shown below, and are available from Digi-Key.
Kilo ML-50-1-6MM gloss clear (Digi-key 226-2003-ND)
Kilo ML-50-2-6MM gloss black (Digi-key 226-1003-ND)
Kilo ML-50-3-6MM matte clear (Digi-key 226-3003-ND)
Kilo ML-50-4-6MM matte black (Digi-key 226-4003-ND)
You may find other knob styles from different vendors usable as well.
The Mini³ has an onboard battery charger designed only to be
used with a 9V NiMH (nickel metal hydride) rechargeable battery.
No other battery types should be used to avoid possible battery explosion.
NOTE: Do not, under any cirmcumstances, use a
Li-ion or non-rechargeable battery in the Mini³. Specific
LiPo batteries are supported only with the optional
χ1 battery management board.
The battery is snapped in place by onboard battery contacts, and the
case has just enough clearance for a standard size 9V battery. However,
not all battery vendors adhere strictly to the standard, and some
batteries may be too thick to fit.
The CTA 325mAH or 275mAH, and the
Accupower 300mAH and
9V NiMH batteries are
recommended for Mini³.
These have high capacity, providing long play times between
charges, has an excellent 500-1000 cycles of recharge life,
and are reasonably priced.
Note that some samples of these batteries have been found to be thicker
than spec, and may not fit properly in the Mini³. In most cases
peeling off the plastic film wrapping and sanding away a little of the
largest flat surfaces of the battery will make them fit.
The above batteries are available from various sources, including
Thomas Distributing and
zbattery.com in the US.
You can try another brand of 9V NiMH battery, but you'll have to check the
Due to Mini³'s onboard voltage regulation, the AC adapter
requirement is not very stringent. It should be a linear adapter
(not switching) to avoid high-frequency noise, but does not need
to be regulated. It must output somewhere between 15VDC and 24VDC.
24VDC is the absolute maximum. Do not trust the label, you should
measure it with a DMM to be sure, because unregulated DC adapters usually
output more voltage than specified). The minimum current rating should
be 300mA to ensure that it could provide all the current the amp might
need under all circumstances.
The AC adapter should be fully isolated. That is, if it has a
three-prong AC wall plug, the AC earth pin should not be internally
connected to either the positive or negative outputs. Using your DMM,
you should verify this by checking the resistance between them.
The readings should both show infinity ("OL").
Many "12VDC" unregulated
DC adapters actually output more than 15VDC, and are good to use
The AC adapter's output plug should be a barrel-type with 2.5mm ID,
5.5mm OD female, the polarity is center-positive. The appropriate
input AC mains voltage, frequency and plug style depends on your
Example adapters are listed in the parts list table above.
The L1, L2 and L3 leaded ferrite beads and the B+ and B- board-mount battery
contacts are supplied with each Mini³ circuit board, so you do
not have to purchase them separately. If, for any reason, you need
to replace them in the future, you may
or, you may purchase replacements elsewhere. Here are the specifics:
Do not substitute the ferrite beads with a different part number. These
beads have been carefully tested and selected amongst many similar products
for the best performance. Using different beads may impact the stability
of the amplifier.